As the SRAM cell is scaled towards the 64 Mb SRAM and beyond, innovative cell concepts are needed to push the cell area to practical limits. When the cell area is scaled down, the storage node capacitance is also reduced, resulting in higher soft error rates. Exposure to naturally occurring radiation can alter the charge stored at the storage (latch) node, and can change the data stored. Fast SRAMs require lower Soft Error Rates ("SER"), because fast dedicated caches tend to rely less on Error Correction Coding ("ECC"), which are required to compensate for high SER's.
Several schemes have been proposed in the past to reduce soft errors in SRAMs. One scheme involves the placement of a stacked capacitor between the input/output and the latch nodes. (See, for example, U.S. Pat. Nos. 5,016,070 and 5,145,799.) However, such a capacitor increases the noise coupling between the input/output and the latch (storage) nodes, thereby increasing the noise susceptibility of the cell. In addition, the capacitance associated with the stacked capacitor is limited, and the stacked capacitor introduces planarization problems.
In accordance with the present invention, the soft error and other stability problems associated with the scaling down of SRAMs to higher and higher densities can be eliminated by placing a trench storage capacitor under each of the latch nodes, and connected to the latch nodes. The capacitor is between the latch node and the ground, and so does not increase noise from inputs. The node capacitance can be arbitrarily increased without increase in cell area by increasing the trench depth. Prior designs have placed a capacitance across the bitline access device in an attempt to reduce soft error rates. This technique has proven to be ineffective, however, because high frequency noise can pass from input/output through such capacitors, resulting in errors. In accordance with the present invention, however, because device stabilization is achieved through a capacitance connected to ground, where no such high frequency noise exists, better cell stability is achieved.
In addition, a novel multi-layer contact proposed here reduces the cell area.